This invention relates generally to surface wave integratable filters (SWIF's) and specifically to those utilizing multistrip couplers (MSC).
SWIF devices generally comprise piezoelectric substrates upon which transmitting and receiving transducer pairs are formed. The transducers, whether receiving or transmitting, are typically formed by pairs of electrically conductive comb-like structures having interleaved fingers. When a voltage is applied between the comb-like structures of a transmitting transducer, the piezoelectric material of the substrate's surface is stressed and deformed causing a conversion of electrical energy to mechanical energy in the form of acoustic surface waves which propagate across the medium surface to impinge the receiving transducer. A second energy conversion takes place at the receiving transducer in which a portion of the mechanical energy of the acoustic surface wave is reconverted to electrical energy, developing a voltage between comb elements.
While a simple single transducer section, formed by two adjacent fingers of one comb element and the interleaved finger of the other, is capable of launching or receiving surface waves, in practice many transducer sections are combined in a more complex transducer structure. In multiple section transducers, such as are generally used, the total launched or received wave is the cumulative effect of the individual transducer sections. Transducers define a single maximum energy or primary propagation and reception axis. While most propagation occurs along this axis, a small but significant amount is radiated in or received from other directions. Generally the primary axis is in essentially orthogonal alignment with the transducer fingers while these other energy transfer directions are non-orthogonal.
When electrically stimulated, transmitting transducers produce both surface waves (propagating at or near the medium surface) and bulk mode waves (propagating deep within the medium). Bulk mode waves are undesirable in surface wave devices because they often reflect off substrate boundaries and impinge the receiving transducer, causing spurious responses which often have a different time delay and exhibit a completely different frequency response.
The deleterious effect on the transfer function caused by the undesired bulk waves may be avoided by offsetting the transmitting and receiving transducers and sidestepping the wave propagation. In sidestepping SWIF devices, the transducers are laterally offset, that is, their primary axes are parallel but not coincident, and an interposed multistrip coupler is positioned orthogonal to the primary axes. Surface waves propagated along the primary axis of the transmitting transducer are "converted" to surface waves along the primary axis of the offset receiving transducer. Simply stated, the sidestepping effect of the multistrip coupler assures that surface waves launched by the transmitting transducer, which are offset by the multistrip coupler, reach the receiving transducer while bulk waves, which are not offset by the multistrip coupler, do not.
The action of such multistrip couplers is best described by analysis of the respective symmetrical and antisymmetrical mode components of the launched acoustic wave as it travels through the coupler. Once the acoustic wave is resolved into symmetrical and anti-symmetrical mode components, their propagation can be considered individually and the total output wave derived by superposition of the individual mode components. Because the multistrip coupler elements are spaced periodically upon the propagating surface, the coupler has an inherent broad bandpass frequency characteristic with a stop band or "notch" for signals having acoustic wavelengths approximately twice the element spacing. An understanding of coupler action is best obtained by analysis of signals outside the coupler stop band. Therefore, the discussions which follow will initially deal with them.
As mentioned above, the primary axes of the transmitting and receiving transducers are parallel, but offset. The individual transducer primary axes define separate "tracks" or channels symmetrical about an imaginary centerline drawn through the coupler orthogonal to its elements. The resultant structure is a first track through one half of the coupler in alignment with the transmitting transducer and a second track through the other half of the coupler in alignment with the receiving transducer.
A symmetrical mode component is a wave component distributed equally along an entire coupler element, that is, the waves in both tracks are in phase. An anti-symmetrical mode component is an acoustic wave having components in each track of equal amplitude and opposite phase. When an anti-symmetrical mode wave is combined with a symmetrical mode wave of the same amplitude, the result resembles an acoustic surface wave in one half of the coupler array only, the two modes having a null effect in the other half. Hence an acoustic surface wave incident upon half the coupler array is effectively divided equally between symmetrical mode and anti-symmetrical mode components.
The anti-symmetrical mode component tends to result in an uneven voltage distribution across the coupler element causing a corresponding element current as the voltage distribution equalizes. In contrast, the symmetrical mode component produces a unifom voltage distribution across the element and no equalizing current is produced. As a result, the anti-symmetrical component has a slower propagation velocity than the symmetrical. As the two mode components advance through the coupler the propagational velocity difference gives rise to an increasing phase displacement between the mode components. In the typical sidestepping device, the coupler is sufficiently long to develop a 180.degree. phase shift.
At this point all the acoustic energy originally emanating from the transmitting transducer propagating in the first track has been "switched over" and now propagates away from the coupler in the second track toward the receiving transducer.
As mentioned, the multistrip coupler has a broad passband response and a notch or stop band corresponding to signals having acoustical wavelengths approximately twice the coupler element spacing. Acoustic surface waves within this range of wavelengths are usually attenuated relative to those outside it. In applications requiring a continuous relatively flat coupler response over an extended range of signal frequencies, element spacing is selected such that the stop band occurs well outside the frequencies of interest. In the more typical applications in which the acoustic device functions as a filter, it is intended to exhibit a predetermined frequency characteristic, passing a selected range of signals while attenuating others. Coupler element spacing in such acoustic filters can be selected such that the stop band corresponds to signals sought to be trapped or attenuated.
If, for example, an acoustic surface wave device is used as the intermediate frequency filter for an NTSC color television receiver, several alternatives arise. Element spacing may be selected to provide a stop band at the lower frequency end of the filter response corresponding to signals at the adjacent picture carrier frequency (39.75 MHz) or the stop band may be located at the higher frequency end of the filter response corresponding to the adjacent channel sound carrier frequency (47.25 MHz). Using this approach, approximately 10 decibels (db) of attenuation within the coupler may be achieved. However, the trap produced is extremely narrow making consistency difficult to achieve. Therefore, while the trap achieved by such use of coupler periodicity has reasonable attenuation, it is prohibitively narrow for typical manufacturing operations.